If you as the supplier and your customer have not discussed and agreed to certain terms then the laws regarding late payments (this applies only to business to business transactions) are applicable.
Those laws state that 30 days from date that the service was provided would be the latest acceptable date in law by which you should be paid.
If you have discussed terms with your client and agreed a longer period then this is fair enough.
However, once the payment terms which you have agreed with your client have been broken you are entitled by law to issue a Late Payment Notice for which you are legally entitled to charge for (£40 for debts under £1000 I believe). Also you are entitled to levy a daily interest charge of 8% over the base rate (currently 0.5%).
Whether you do this or not is completely up to you! You may decide that future business prospects with your client may be worth more than the £40 plus interest and decide not to pursue it, but the best thing is that you are not legally obliged to pursue it nor does non pursuance affect your rights under this law.
For any invoice which was paid late you have up to 6 years from the date which the invoice was due to be paid to pursue this.
I had to chase up 4 invoices from November last year just this week. 1 to a company who have used us constantly over the last two years and who have not only pretty much always paid on time, there are times when they have paid us early at our request. The other 3 to a company who have used us since we began.
The relationships we have with these companies is so good that I would not ever dream of issuing a late payment notice to them.
On the other hand there is another company who requires our services every once in a while yet constantly pays up to a week late on net monthly terms. Payment always needs to be chased and the personnel in accounts are rather off hand when you are just politely trying to inform them that they are late AGAIN! For companies such as this I keep a written record of how late each invoice was paid, the amount of £40 late payment charges they now owe (one per invoice) plus the interest due for each invoice.
I'll add it up once we are close to the 6 years and see if it's worth pursuing :-)
Oddly enough one minimum charge job was paid so late that the total late payment charge plus interest for this job would (if I desired to charge it) be almost 15 times the original price of the job it's self.
In short, you have the legal right to make a late payment charge plus interest, but only you can decide whether that is more financially viable for you or not.